The present invention relates to a code division multiple access (CDMA) communication system and, more particularly to a receiver having a correlation filter in such a CDMA communication system.
Code Division Multiple Access (CDMA) modulation, which is known in the art, is a multi-user access transmission scheme in which signals from different users overlap 20 both in frequency and in time. This is in contrast to Frequency Division Multiple Access (FDMA), also known in the art, in which user signals overlap in time, but are assigned unique frequencies, and Time Division Multiple Access (TDMA) in which user signals overlap in frequency, but are assigned unique time slots. CDMA signaling is frequently used in cellular communication systems between a base station (BS) within a cell and a plurality of mobile stations (MS) in the possession of users within the cell. The CDMA transmitted signal for each user that broadcast from the user""s mobile station (MS) is spread over a wide bandwidth, which is greater than the initial user information bandwidth. Each user""s signal is spread by a different spreading code to create a wideband spread. All of the spread wideband signals transmitted by the different users are received at the base station (BS) and form a composite received signal. The receiver at the base station (BS) distinguishes different users by using a local copy (or local reference) of the spreading code, which is available to both the mobile stations and the base station in the CDMA system. Such a process is called channelization. In an exemplary CDMA system according to the IS-95 standard and which is well known in the art, channelization in the reverse link, ie., when a mobile station (MS) is transmitting to a base station (BS) in the system, is accomplished using a wideband code called a pseudorandom noise (PN) code, also known in the art. The receiver at the base station (BS) sifts the desired signal from a particular user out of the composite signal by correlating, i.e., using a correlation filter (CF), on the composite signal with the original wideband code. All other signals having codes that do not match the code for the desired user code are rejected.
An exemplary CDMA wireless system includes a plurality of data channels, e.g., the access and traffic channels (and more channels depending on the design of the CDMA system). In the reverse link, the traffic channel is used to transmit user data and voice, as well as signaling messages. The access channel is used by the mobile station (MS), e.g., a cellular phone, to communicate control information with the base station (BS) in the wireless system when the MS does not have a traffic channel assigned. In particular, the MS uses the access channel to make call originations and to respond to pages and orders. These data channels in the CDMA system have different functions and data rates. A receiver in the MS designed to accommodate data transmission in the different channels requires various types of correlation filter (CF) and digital signal processing (DSP) designs for different data rates. Such requirements contribute to the complexity and increase the cost of the receiver design.
There is, therefore, a general need in the art for a wireless system with a flexible, non-complex receiver design. A wireless system is particularly needed that provides a single correlation filter (CF) in the receiver which can be used in receiving data in all data channels. There is a further need for a receiver design with a correlation filter that serves all data channels and is also DSP programmable, which enhances system flexibility.
The invention relates to a reverse link receiver in wireless systems and a correlation filter thereof. A transmitter and a receiver are provided in the reverse link of a wireless system according to the invention. The receiver includes (1) a field programmable gate array (FPGA) which comprises a pseudorandom noise (PN) code generator, (2) a pilot post processor, (3) a data post processor (4) a correlation filter (CF), and (5) a digital signal processor (DSP). The field programmable gate array (FPGA) and the correlation filter (CF), along with the digital signal processor (DSP), are included in the receiver to recover the original data transmitted by the transmitter. The correlation filter (CF) comprises a (correlating filter) CF core for processing data in the three channels, namely the access, maintenance and traffic channels. The DSP is used to control and post-process the outputs of the field programmable gate array FPGA. A mode controller in the DSP controls the channel selection (from the access, maintenance and traffic channels) and the channel symbol mode (data and/or pilot). The pilot post processor and data post processor in the FPGA, in conjunction with the DSP, provide pilot symbol aided QPSK demodulation of up to 3 multipaths received at the receiver. QPSK modulation is a modulation technique that allows the transmission of two bits of information in each symbol period. QPSK modulation makes use of the quadrature component I in addition to the in-phase component Q of a symbol in the frame being transmitted from the transmitter to the receiver. The I and Q components are typically viewed as the real and imaginary parts of a complex signal being transmitted in the channels of the CDMA system. In QPSK, the in-phase component, I, and the quadrature component, Q, can be combined without interfering with each other (i.e., they are orthogonal to each other) which doubles the bandwidth efficiency in comparison with simply transmitting one bit of information in a symbol period. Using time multiplexing, the CF core provides pilot symbol correlation at the three data or chip rates (tiers 1, 2 and 3). A chip is a unit of time which corresponds to the output interval of the PN spreading code. The chip time determines the bandwidth of the CDMA waveform and the chip time divided by the user symbol time determines the spreading factor of the system. For example, the sampling period for a chip in CDMA standard IS-95, known in the art, is {fraction (1/1228800)} seconds. The pilot post processing and data post processing in the FPGA, in conjunction with the data post processor and pilot post processor in the DSP, provide QPSK demodulation and recovery of the original data transmitted by the transmitter for all three channels, i.e., the access, maintenance and traffic channels. The CF core according to the invention performs 8-chip (i.e., tier 1 rate) complex correlation with 64 correlation lags while allowing no data loss as the 64 correlation lags are being processed, where a lag is a time instant for which the PN code is held constant so that outputs can be generated. This is advantageously achieved with a single time-multiplexed 8-chip correlator engine (i.e., CF core). The CF core, which is an 8-chip correlation engine, is time multiplexed to allow multiple 8-chip correlations to be performed with the same correlation engine. Furthermore, the invention advantageously provides the ability to generate integer multiples of the 8-chip correlation for larger correlation lengths, e.g., 32 chips (tier 2) or 128 chips (tier 3). The CF design according to the invention, in conjunction with the DSP, provides temporal diversity of the data signals by combining CF outputs. Spatial diversity is also achieved by providing a plurality of correlation filters in accordance with the CF design of the invention. Diversity is a technique employed to avoid or mitigate the negative effects of fading and interference. Diversity generally refers to the ability of a communication system to receive data or information via several independently fading channels. In general, diversity enhances a receiver""s ability to combine or select (or both) data signals arriving from these independently fading channels, thereby enabling (or facilitating) the extraction of data channels. A particular type of diversity is temporal or time diversity, where the same data signals transmitted in different multipaths and received at different time points at the receiver provide the diversity needed for combining or selecting the data signals. An exemplary diversity technique is maximum ratio combining, or MRC, known in the art. MRC provides sequences of weights in the multitude of data channels in the communication system. A sequence of distinct weights is assigned to segments of a data signal being transmitted. Copies of the data signal are produced for the antennas used in transmitting the data signal. This gives rise to temporal diversity at the receiver when copies of the data signal are received. With a single, common correlation filter design, the invention advantageously provides temporal diversity for all the data channels and modes without the necessity of using different correlation filters for different modes or channels. Temporal diversity is achieved by providing three outputs from the different multipaths and combining the three outputs into one, e.g., using MRC.
According to an embodiment of the method of the invention, a single, common correlation filter (CF) design is provided in a wireless system using CDMA. A plurality of channels with different data rates are provided in the wireless system. The channels provided in the wireless system include the access channel, the maintenance channel, and the traffic channel in which information (e.g., pilot or data symbols or both) is transmitted at the tier 1, tier 2 and tier 3 rates. The data rate for transmitting the information is programmable by the digital signal processor (DSP). A user-unique code, such as a PN code, is applied to the information being transmitted in the channels of the wireless system. The information is QPSK modulated and transmitted in any one of the channels and at any data rate. The transmitted information is correlated at the smallest data rate (i.e., the tier 1 rate) using time multiplexing in the correlation filter (CF) of the wireless system. The correlated information is then demultiplexed and QPSK-demodulated. The demodulated information is summed at the proper integer multiple of the tier 1 rate to achieve the tier 2 and tier 3 rates if needed. The three strongest multipaths (in terms of the received power) are selected in a window or time period for optimal information recovery. Furthermore, three outputs from the demodulated information can be provided and combined for temporal diversity. Spatial diversity is achieved by providing the single, common correlation filter design in a plurality of receivers in the wireless system. All the process steps according to the invention described herein are advantageously accomplished using a single, common correlation filter (CF) design, which eliminates the need for additional correlators or correlation filters for processing received information having multiple data rates.